This proposal concerns the roles of Axin and the related gene Axin2 in regulating the canonical Wnt signal transduction pathway during mammalian embryogenesis. The Wnts are a family of secreted factors that play important roles in cell proliferation, patterning and differentiation during development. Axin is a critical component of a protein complex that controls intracellular signaling downstream of Wnts, by regulating the levels of ?-catenin, a transcriptional co-activator and a key effector in the pathway. Axin2 is believed to have a similar function, although it is not as well characterized. Mutations in both genes in humans have shown that they are tumor suppressors, consistent with their roles in negatively regulating the Wnt pathway. The analysis of mice with mutations in Axin and Axin2 have revealed that these genes are important for early axial patterning, as well as for craniofacial and brain development. Craniofacial development is a complex process that involves interactions between the surface ectoderm, endoderm, mesoderm and the neural crest, and it is highly susceptible to genetic and environmental perturbations, as craniofacial abnormalities are among the most common birth defects in humans. The Wnt signaling pathway has been implicated in craniofacial development through several mouse mutations. In this proposal, the hypothesis will be tested that Axin and Axin2 are required to negatively regulate the response to certain Wnts by the cranial neural crest cells that form much of the face, and that mutations in Axin and Axin2 lead to inappropriate activation of the Wnt pathway. This will be addressed by analysis of mutant mice, as well as through the conditional manipulation of the Wnt pathway in the cranial neural crest, using transgenic approaches. To understand how Axin and Axin 2 cooperate during development, it is important to define the extent to which they are functionally redundant, and this issue will be investigated through a gene replacement strategy in mice. Finally, to test in vivo the importance of specific domains of Axin that are believed to mediate its functional interactions with other components of the pathway, several novel mutant alleles of Axin will be generated, and examined for their effects on mouse embryogenesis.